Manufacture of solid state capacitors

ABSTRACT

The present invention concerns the field of solid state capacitors and relates particularly to massed production methods for manufacturing solid state capacitors. According to one aspect of the invention there is provided a method of manufacturing multiple solid state capacitors comprising: providing a substrate layer; forming on an upper surface of the substrate layer a plurality of upstanding bodies consisting of porous sintered valve-action material; forming an insulating layer on and extending through the porosity of the bodies; forming a conducting cathode layer on the insulating layer; and forming cathode termination means on a top portion of each body; and dividing the processed substrate to form a plurality of individual capacitor bodies in each of which a portion of the substrate forms an anode terminal, a porous body is an anode body having cathode termination means provided on a top portion thereof, characterized in that the upstanding porous bodies are formed by pressing valve-action metal powder/binder mixture onto the substrate and forming the required body shapes and distribution while the mixture is in the green state and subsequently sintering the bodies.

The present invention concerns the field of solid state capacitors andrelates particularly to massed production methods for manufacturingsolid state capacitors.

BACKGROUND OF THE INVENTION

A massed production method for solid state tantalum capacitors isdescribed in U.S. Pat. No. 5,357,399 (inventor Ian Salisbury). Thismethod involves providing a substrate wafer of solid tantalum, forming asintered, highly porous, layer of tantalum on the substrate, sawing thelayer of porous tantalum with an orthogonal pattern of channels toproduce an array of upstanding porous tantalum rectilinear bodies,anodising the bodies to form a dielectric layer thereon, dipping thebodies in manganese nitrate solution and heating to convert the appliedsolution to manganese dioxide thereby to form a cathode layer, applyingrespective conducting layers of carbon and then silver onto top ends ofeach body, bonding a lid consisting of a wafer of solid metal onto thesilver layer; injecting insulating resin material into the channelsbetween bodies constrained by the substrate and lid; and slicing theassembly in a direction perpendicular to the plane of the wafers andalong the centre line of each channel thereby to produce a plurality ofcapacitors in which the anode terminal consists of substrate material,the cathode terminal consists of lid material and the capacitive bodyconsists of the coated porous tantalum body.

Our pending application PCT/GB99/03566 (corresponding to UK applicationnumber 9916047.5), describes a variation on the Salisbury method, inwhich the lid layer is omitted. In capacitors manufactured according tothis method the volumetric efficiency of each capacitor produced ismaximized.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved methodof manufacturing solid state capacitors, particularly a method which issimpler and generates less waste material.

According to one aspect of the present invention there is provided amethod of manufacturing multiple solid state capacitors comprising:

-   -   providing a substrate layer;    -   forming on a surface of the substrate layer a plurality of        upstanding bodies consisting of porous sintered valve-action        material;    -   forming an insulating layer on and extending through the        porosity of the bodies;    -   forming a conducting cathode layer on the insulating layer;    -   and forming cathode termination means on a top portion of each        body; and    -   dividing the processed substrate to form a plurality of        individual capacitor bodies in each of which a portion of the        substrate forms an anode terminal, a porous body is an anode        terminal with cathode termination means provided on a top        portion thereof, characterised in that the upstanding porous        bodies are formed by (i) providing a mixture of valve-action        material powder and binder, (ii) configuring the mixture into a        plurality of bodies on the substrate while the mixture is in the        green state and (iii) thereafter sintering the bodies.

The green forming may involve a pressing process for example with theuse of a combination die and punch set in which a die is formed with aplurality of holes corresponding to the desired body shapes anddistribution and a plurality of punches are used to press green mixtureplaced in the holes onto the substrate. The die pressing process may berepeated to build up layers of green material in each individual body.

In preferred embodiments the valve-action material is a metal,especially tantalum. However other valve-action metals and materials maybe used in the process of the present invention. Examples of suitablematerials are niobium, molybdenum, silicon, aluminium, titanium,tungsten, zirconium, and alloys thereof and in addition valve actionoxide materials. Particularly preferred examples of suitable metals areniobium and tantalum.

When the valve action metal is tantalum the substrate is preferably asolid tantalum wafer, thereby ensuring physical and chemicalcompatibility with the porous metal. In general, the substrate should becompatible with the valve-action material. Typically the substratematerial will consist of solid valve action material.

A seeding layer of coarse grade powder may have to be applied to thesubstrate and sintered thereto before finer grade green powder/bindermixture is pressed onto the substrate to form the upstanding bodies. Thecoarse grade powder provides mechanical keying ensuring that a strongconnection between the sintered porous bodies and the substrate isproduced. The strong connection is necessary to ensure that separationof the porous bodies from the substrate does not occur during subsequentsteps in the manufacturing process.

Preferably, the seed layer is only laid down on the substrate inpositions where the porous bodies will stand. Hence, according toanother aspect of the invention, there is provided a method ofmanufacturing multiple solid state capacitors comprising:—providing asubstrate layer;—forming on a surface of the substrate layer a pluralityof upstanding bodies consisting of porous sintered valve-actionmaterial;—forming an insulating layer on and extending through theporosity of the bodies;—forming a conducting cathode layer on theinsulating layer;—and forming cathode termination means on a top portionof each body; and—dividing the processed substrate to form a pluralityof individual capacitor bodies in each of which a portion of thesubstrate forms an anode terminal, a porous body is an anode terminalwith cathode termination means provided on a top portion thereof,wherein before formation of the valve-action bodies on the substrate aseeding layer of relatively coarse grain material is provided on thesubstrate, characterised in that the seeding layer is provided as anarray of seeding tabs is formed on the substrate.

Preferably the tabs comprises the same valve-action material as theupstanding bodies. The tabs may be formed by application of a screenprinting layer having the desired distribution of tabs. Alternativelystencilling may be use to lay down the appropriate pattern of tabs. Themethod is not limited to these two and may include other methods ofapplying a suitable array of seeding material tabs. An advantage oflaying down an array of seeding tabs is that the conventional machiningof a uniform seeding layer is bypassed. In addition waste of the seedinglayer material is reduced, with an accompanying economic andenvironmental benefit.

In a preferred embodiment, the tabs are laid down by screen printing ofa paste of relatively coarse valve action material powder in therequired pattern onto the substrate.

After laying down, the array of tabs is typically then sintered to fixthe pattern of seeding tabs in place.

It is not strictly necessary to lay down an array of tabs in the methodof the present invention, and in an alternative embodiment, a uniformlayer can be applied to the substrate, fixed by sintering and thenmachined to remove selectively seeding layer in order to produce therequired pattern of seeding tabs.

The cathode termination means may comprise one or more conducting layersapplied to a top portion of the upstanding bodies. the cathodetermination means may comprise metal plate portions applied to eachupstanding body.

In one embodiment the termination means comprises a solid valve actionmetal lid applied to sandwich the upstanding bodies between the lid andthe substrate. The lid is preferably divided along with the substrate toform the individual capacitor bodies in each of which the lid portion isa cathode terminal. Such a method is fully described in U.S. Pat. No.5,357,399 and is not further described herein.

In another embodiment the lid is omitted and conducting materialtermination layers are applied to the top end regions of the upstandingbodies to form the cathode terminals of each capacitor.

Space between the upstanding bodies on the substrate is preferablyfilled with an insulating material. In one preferred embodiment thespace is filled with an insulating plastics resin material, such asepoxy resin. In this way when the substrate is divided each capacitorbody may be left with a protective resin sleeve about the porous bodyportions thereof.

According to a further aspect of the invention there is provided acapacitor produced by any method hereinbefore described.

According to another aspect of the invention there is provided anelectronic or electrical device comprising a capacitor made by anymethod hereinbefore described.

The dielectric layer may be formed by an electrolytic anodizationprocess in which an oxide film is carefully built up on the surface ofthe porous sintered anode body. Suitable methods will be known to theperson skilled in the art.

The cathode layer may be formed by dipping the anode bodies into acathode layer precursor solution such as manganese nitrate and thenheating to produce a cathode layer of manganese dioxide. Repeateddipping and heating steps may be carried out in order gradually to buildup the required depth and integrity of cathode layer.

Typically, during the dipping process the cathode layer will be built upnot only on the anode bodies, but also on the exposed tantalum substratesurface between bodies. In order that each cathode terminal is isolatedfrom its respective anode terminal a further process step may be carriedout to remove any cathode layer (and dielectric layer) from thesubstrate around each anode body. This process may involve a furthermachining process in which isolating channels are formed between eachanode body by removal of a surface layer of substrate. For example,where orthogonal rows have been machined to form rectilinear anodebodies, isolating channels may be machined along the centre lines of therows and columns between anode bodies. In this way, a step is formed inthe perimeter of each capacitor anode body, which step has an un-coatedsurface, thereby isolating the cathode layer from the exposed anodeterminal.

With the application of the cathode layer, the anode body becomes acapacitive body comprising an anode portion consisting of aninterconnected matrix of: metal powder; dielectric insulating layer ofmetal oxide; and a conducting cathode layer of doped oxide.

The encapsulating resin may be applied under pressure or by simpleimmersion depending upon the suitability and fluidity of the particularresin. Once the resin has set, the resin and substrate may be machinedor otherwise cut to separate adjacent capacitor bodies. Theencapsulation material may be a plastics resin, such as epoxy.

Following is a description by way of example only and with reference tothe drawings of one method of putting the present invention into effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 3 to 5 are cross-sectional views of a substrate duringprocessing according to one embodiment of the present invention.

FIG. 2 is a view from above of the substrate after green forming step inthe process.

FIG. 6 shows sectional view from one side of a capacitor producedaccording to the method of the present invention.

FIG. 7 shows a press to form the upstanding bodies.

BRIEF DESCRIPTION OF THE PREFERRED INVENTION

A transverse section through a solid tantalum wafer is shown as 10 inFIG. 1. An upper surface of the wafer has sintered thereon a pluralityof seeding tabs 12. The seeding tabs comprise a thin layer of coarsegrained capacitor grade tantalum powder 12. The seeding layer ispreferably applied by screen printing of a green paste of coarse powderin an orthogonal pattern (or “check”) of rectilinear tabs. The tabs arethen sintered to fix to the substrate. A green (i.e. un-sintered)mixture of fine-grained capacitor grade tantalum powder is then pressedonto the upper surface of the substrate to form a green layer 13. Apunch and die combination press is used to shape the flowable greenmixture to form the upstanding rectilinear bodies 16, shown in FIG. 2.

The press is shown in FIG. 7. The press comprises a bottom die 40 andseries of punches 41. The die is formed with an array of rectangularsection holes 42, arranged in rows and columns corresponding to theintended distribution of anode bodies on the substrate. The die holesare filled with green tantalum mixture 44. The substrate 10 is thenplaced on top of the die, with the seeding tabs (not shown) aligned withthe holes. A top patten 43 is then clamped down onto the substrate asshown in FIG. 7B. The clamped assembly is pressed down onto the punches.The punches compress the charge of green mixture in each hole and attacheach charge to the substrate. A mechanical stop (not shown) is used tolimit die travel, and prevent over-densification of the green charges.The plattern is then released and separated from the die. A series oflight springs 45 which support the weight of the die, push the die andsubstrate away from the punches on release of the plattern, leaving anarray of green rectangular section bodies attached to the substrate, asshown in FIG. 2. The pressing process may be repeated to produce therequired height of green body by building up layers of green material.

The green bodies are sintered to fuse the fine grained powder into anintegral porous network in each body. The sintering is carried out ataround 1600 degrees centigrade (the optimum temperature will depend uponthe grain size and the duration of the sintering process). The sinteringprocess also fuses the porous bodies to the coarse seeding layer 12.

The formed bodies define an orthogonal grid of transverse channels 14and longitudinal channels 15 as shown in FIG. 2.

The porous bodies form the anode portions of the capacitors. Aninsulating dielectric layer (not shown) is applied to the anode bodiesby anodizing in an electrolyte bath (of e.g 0.1% phosphoric acidsolution) while connecting the positive terminal of a D.C. power supplyto the substrate. This results in the formation of a thin tantalumpentoxide layer on the metal porous surface of the bodies and exposedsubstrate.

A cathode layer (not shown) is then formed on the anode bodies by thewell known manganization process. In this process the anodized anodebodies 16 are immersed in manganese nitrate solution to leave a coatingof wet solution on each body and covering its internal porosity. Thesubstrate is heated in moist air to convert the coating of nitrate tothe dioxide. The immersion and heating cycles may be repeated as many as20 times or more in order to build up the required coherent cathodelayer.

In order to ensure that any dielectric or cathode layer formed on thesubstrate surface perimeter of each anode body is isolated, a furthermachining step is carried out in which an orthogonal pattern of channels32 is sawn into the substrate surface, along the centre lines separatingeach anode body.

Once the manganization is complete the manganized bodies are coated withan intermediate layer 27 of conducting carbon by dipping into a bath ofliquid carbon paste. After the carbon layer has set, a furtherintermediate layer 21 of silver is coated onto the carbon layer bydipping of the carbon-coated bodies into a liquid silver paste. Thesilver layer is not allowed to pass beyond the carbon layer 27 in orderto ensure that silver does not directly contact the incompatible oxidelayer. The silver layer 21 is then allowed to set solid.

A solid sheet 9 of tantalum is then coated on one surface thereof with alayer 5 of PTFE as a release agent. A uniform layer of silver paste 22is then applied to the exposed surface of the PTFE. The sheet is thenplaced silver-side down onto the top ends of the bodies 16 to from a lid9 shown in FIG. 4.

Downwards pressure is applied to a top side of the sheet in order toforce the immobilized paste 22 to flow into intimate adhesive contactwith the intermediate silver layer 21. In addition the contact isfurther enhanced by the paste flowing to a small extent down the sidewalls of each capacitor, but not beyond the carbon layer.

With the lid in place, the channels 14,15 between the capacitor bodiesare filled with liquid epoxy resin 20 as shown in FIG. 4. The resinsurrounds the sides of each capacitor body, up to the level of the lidpaste 22. The channels are filled by injection under pressure of theresin, thereby ensuring complete filling of the space defined by thechannels. The structural constraint provided by the tantalum lid 9maintains the integrity of the intermediary layers 27,21 and 22 duringthe encapsulation process.

When the resin 20 has set, the lid sheet is removed. The PTFE layer 5readily separates from the set silver layer 22, leaving the upper endregion of each body coated in a solid silver layer. The presence of thelid 9 ensures that a flat top surface layer 22 is formed after removalof the lid, as shown in FIG. 5.

The wafer may now be sliced along the centre line (shown as dashed linesin FIG. 5) of each channel 14,15 in order to separate each capacitorbody from its neighbours. The resulting individual capacitor structureis shown in FIG. 6. Each capacitor consists of an anode terminal portion23 consisting of the substrate material. Upstanding from the substrateis the capacitor body 16 which is sheathed in epoxy resin sidewalls24,25. The step 30,31 in the substrate corresponds to the machineisolation channels 32 formed in the original substrate wafer. This stepis free of manganized coating and any other contaminant, and thereforeensures that the exposed anode terminal is isolated from the cathodeterminal. The top end region of each capacitor is coated in a layer ofcarbon paste 27, a layer of silver paste 21 and a further layer ofsilver paste 22 which forms a cathode terminal portion of the component.

A final processing stage is a five-sided termination process. This is awell known process in the electronics industry which involves theformation of end caps 28,29 which form the external terminals of thecapacitor. The termination layer metal may consist of discrete layers ofsilver, nickel and tin (preferably in that order). These are suitablemetals for forming electrical connections by soldering of the capacitorterminals to contacts or other components of an electrical or electroniccircuit.

The present invention is an elegant adaptation of the previous knownprocess which provides a significant improvement in the economicefficiency of the process. The grinding steps of known processes, andtheir inevitable production of waste tantalum powder, is bypassed. Thissimplifies the production machinery by removing the need for theexpensive grinding apparatus. Instead of grinding away sintered porouslayer to form the bodies the powder/binder mixture is formed whilegreen. In this case it is easy to re-use or recycle excess greenmixture.

1. A method of manufacturing multiple solid state capacitors comprising:providing a substrate layer; forming on a surface of the substrate layera plurality of upstanding bodies consisting of porous sinteredvalve-action material by (i) providing a green mixture of a firstvalve-action material powder and binder, (ii) pressing of the greenmixture onto the substrate and configuring the mixture into a pluralityof bodies on the substrate while the mixture is in the green state and(iii) thereafter sintering the bodies; forming an insulating layer onand extending through the porosity of the bodies; forming a conductingcathode layer on the insulating layer; forming cathode termination meanson a top portion of each body; and dividing the processed substrate toform a plurality of individual capacitors for each of which one endcorresponding to the substrate forms an anode terminal, a porous body isa capacitive portion and an opposite end is provided with cathodetermination means to form a cathode terminal, characterized in that thepressing and configuring of the green mixture into a plurality of bodiesis conducted by means of a die and punch combination in which the die isformed with an array of holes corresponding to the required bodysections and body distributions, green mixture is placed into theplurality of holes and a plurality of punches entered into the holeswhile the die and substrate are provided in adjacent contact with oneanother press a green mixture charge in each hole onto the substrate,after which point the die and substrate are separated leaving theplurality of upstanding bodies.
 2. A method as claimed in claim 1wherein the die/punch pressing process is repeated to build up furtherlayers of green mixture in each body.
 3. A method as claimed in claim 1or claim 2 wherein a seeding layer of a second valve action materialpowder is applied to the substrate and sintered thereto before thebodies are pressed and configured onto the substrate, thereby to providemechanical keying between the porous bodies and the substrate.
 4. Amethod as claimed in claim 1 or claim 2 wherein an array of seedingtabs, each tab comprising a sintered layer of a second valve actionmaterial powder, is formed on the substrate in a pattern correspondingto the intended position of the green bodies, and the green bodies areformed on the seeding tabs, thereby to enhance the bond between eachbody and the substrate.
 5. A method claimed in claim 4 wherein theseeding tabs are applied by screen printing of a paste of valve-actionpowder in a pattern corresponding to the array, and the paste issintered to fix the pattern in place on the substrate.
 6. A method asclaimed in claim 4 wherein the seeding tabs are applied by applying alayer of a paste of valve-action powder over a stencil to form a patterncorresponding to the army, and the paste is sintered to fix the patternin place on the substrate.